The absence of simple examples of superconductivity adjoining itinerant-electron ferromagnetism in the phase diagram has for many years cast doubt on the validity of conventional models of magnetically mediated superconductivity. On closer examination, however, very few systems have been studied in the extreme conditions of purity, proximity to the ferromagnetic state and very low temperatures required to test the theory definitively. Here we report the observation of superconductivity on the border of ferromagnetism in a pure system, UGe2, which is known to be qualitatively similar to the classic d-electron ferromagnets. The superconductivity that we observe below 1 K, in a limited pressure range on the border of ferromagnetism, seems to arise from the same electrons that produce band magnetism. In this case, superconductivity is most naturally understood in terms of magnetic as opposed to lattice interactions, and by a spin-triplet rather than the spin-singlet pairing normally associated with nearly antiferromagnetic metals.

Unlike the parent phases of the iron-arsenide high-Tc superconductors, undoped FeSe is not magnetically ordered and exhibits superconductivity with Tc approximately 9 K. Equally surprising is the fact that applied pressure dramatically enhances the modest Tc to approximately 37 K. We investigate the electronic properties of FeSe using 77Se NMR to search for the key to the superconducting mechanism. We demonstrate that the electronic properties of FeSe are very similar to those of electron-doped FeAs superconductors, and that antiferromagnetic spin fluctuations are strongly enhanced near Tc. Furthermore, applied pressure enhances spin fluctuations. Our findings suggest a link between spin fluctuations and the superconducting mechanism in FeSe.

We report the first NMR investigation of spin dynamics in the overdoped nonsuperconducting regime of Ba(Fe1-xCox)2As2 up to x=0.26. We demonstrate that the absence of interband transitions with large momentum transfer Q{AF} approximately (pi/a,0) between the hole and electron Fermi surfaces results in complete suppression of antiferromagnetic spin fluctuations for x greater than or approximately 0.15. Our experimental results provide direct evidence for a correlation between T{c} and the strength of Q{AF} antiferromagnetic spin fluctuations.

We report a systematic investigation of Ba(Fe 1Àx Co x ) 2 As 2 based on transport and 75 As NMR measurements, and establish the electronic phase diagram. We demonstrate that doping progressively suppresses the uniform spin susceptibility and low frequency spin fluctuations. The optimum superconducting phase emerges at x c ' 0:08 when the tendency toward spin ordering completely diminishes. Our findings point toward the presence of a quantum critical point near x c between the SDW (spin density wave) and superconducting phases.KEYWORDS: iron pnictide superconductor, high temperature superconductivity, NMR, quantum criticality DOI: 10.1143/JPSJ.78.013711The recent discovery of iron-pnictide high T c superconductors 1-6) poses a new intellectual challenge in condensed matter physics. Although the superconducting mechanism remains unknown, it has become apparent that ironpnictides share remarkable similarities with high T c cuprates: the undoped parent phases LaFeAsO and BaFe 2 As 2 have FeAs square-lattice sheets, and itinerant electrons in these layers undergo antiferromagnetic long range order (AFLRO) below modest temperatures, T SDW $ 140 K; [7][8][9] electron or hole doping suppresses the AFLRO and induces superconductvity. There exist clear dissimilarities, too. For example, all five 3d orbitals of Fe atoms contribute to the multiple Fermi surfaces, hence to superconductivity, in ironpnictides. 10,11) In contrast, only the Cu 3d x 2 Ày 2 orbital plays a role in curates. Furthermore, substitution of Co atoms into Fe sites of the parent phases results in electron doping and induces superconductivity [12][13][14] without creating localized moments, 15) while Zn 2þ ions doped into Cu 2þ sites induce localized moments and destroy superconductivity in cuprates. Sorting out these similarities and dissimilarities may lead us to an understanding the mechanism of high T c superconductivity in iron-pnictides as well as in cuprates.In this letter, we utilize transport and NMR techniques to probe the evolution of the electronic properties of Ba(Fe 1Àx -Co x ) 2 As 2 single crystals from the undoped SDW phase (x ¼ 0 with T SDW ¼ 135 K), underdoped SDW phase (x ¼ 0:02 with T SDW ¼ 100 K, and x ¼ 0:04 with T SDW ¼ 66 K), optimally doped superconducting phase (x ¼ 0:08 with T c ¼ 22 K), to the slightly overdoped regime (x ¼ 0:105 with T c ¼ 15 K). We establish the electronic phase diagram which has a quantum critical point near the optimum concentration x c $ 0:08, and explore the possible relation between paramagnetic spin fluctuations and the mechanism of superconductivity. From NMR Knight shift and spin-lattice relaxation rate measurements, we show that electron doping progressively suppresses the uniform spin susceptibility spin and low frequency spin fluctuations. The optimally doped superconducting phase emerges at x c $ 0:08 as soon as doped electrons completely suppress the tendency toward AFLRO.We grew single crystals of Ba(Fe 1Àx Co x ) 2 As 2 from FeAs flux, 13) and determined the actual Co concentration x by energ...

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